When information is transmitted in digital form from a transmitter to a receiver, a method is commonly used in which the information to be transmitted is divided into data frames. In addition to the actual information field, data frames can comprise also other elements, such as synchronization data fields and other control data fields. Further, a data frame can include e.g. parity control information for securing the accuracy of the data transmission. In this specification, the information field of the data frame will be called the information element and the other data fields will be called jointly the control element.
In data transmission between a mobile station and a base station, the conditions vary upon moving of the mobile station as well as upon changes in environmental and atmospheric conditions. Thus the data transmission is susceptible of disturbances, wherein the received information may contain errors. For elimination of errors e.g. in the GSM system, channel coding is used at the transmission stage and channel decoding accordingly at the receiving stage. In channel coding, error correction data is added to the information to be transmitted for example by generating parity control bits by an error correction algorithm and adding them to the information to be transmitted. At the receiving stage, the operation is inverse, wherein possible errors can be detected and the incorrect data corrected with the error correction algorithm, if the number of errors is smaller than or equal to the error correction capacity of the error correction algorithm.
In addition to the above-mentioned changes in conditions, problems are caused in data transmission between the mobile station and the base station also by a delay in the propagation of signals from the transmitter to the receiver. Because the distance between the mobile station and the base station changes, the signal propagation delay will change accordingly. For example in the GSM system, which is a so-called cellular system, the maximum radius of a cell is relatively long, of the order of 35 km. Thus also the propagation delay can vary to a significant degree, wherein the synchronization between the mobile station and the base station becomes difficult.
For the base station to operate as efficiently as possible, it must receive the information transmitted by the base stations in a predetermined time. For compensation of the propagation delay between the mobile station and the base station e.g. in the GSM system, so-called timing advance is applied. However, the length of the timing advance is not known until the mobile station has formed a connection to the base station. The timing is determined by means of a random access burst transmitted by the mobile station e.g. according to Slotted Aloha protocol. During transmission of the burst, the reception by the base station is not accurately synchronized, wherein the access burst comprises a relatively long synchronization bit sequence (synchronization data field). After the burst, there is still an extended guard period of 63.25 bits intended for eliminating the effect of the propagation delay between transmission and reception, i.e. overlapping of successive bursts upon reception.
FIG. 2 shows an access burst according to GSM Standard 05.05. The burst comprises a starting data field (Extended Start) of 8 bits, a synchronization data field (Sync Sequence) of 41 bits, an information field (Encrypted Data) of 36 bits, and a terminating data field of 3 bits. The length of the guard period after the access burst is 68.25 bits. For increasing the probability of accurate reception, the bits are subjected to channel coding by so-called convolution coding which is also presented in GSM standard 05.05. The coding is conducted with a bit ratio of 1/2, wherein two bits are formed for each bit to be coded. Thus the number of information bits available is 18, including six parity bits and four tail bits. After this, only eight bits are available for user information. Part of these user information bits are used for informing the base station about a random number, wherein different mobile stations can be separated from each other at the base station. The rest of the user information bits are used e.g. for indicating the establishment cause of the message from the mobile station. For example in a bit sequence "101XXXXX", the binary digits 101 indicate that the establishment cause is an emergency message and the sequence XXXXX, where X can be either binary digit (0/1), indicates the random number selected by the mobile station. Thus the base station transmits as a response to the mobile station an acknowledgement message containing e.g. said random number, wherein the mobile station recognizes (with a certain probability) that the message is intended for it.
When the mobile station sends the base station a request for a channel for communication, the mobile station transmits a channel request message to the base station. The message contains also said random number. Since the number of bits available for the random number is limited (5 bits in the above-mentioned example), this may cause problems particularly in situations where there is a large number of channel request messages. For example in packet radio networks, each packet to be transmitted will usually require a channel request message.
After channel coding, the coded bits are formed into a burst to be transmitted and conveyed to a modulator. FIG. 1a is a reduced block diagram showing such a method for transmitting an access burst according to the prior art. In a corresponding manner, FIG. 1b is a reduced block diagram showing the reception of a burst according to the prior art.
For enhancing the efficiency of information transmission, methods have been developed for changing the information to be transmitted in a denser, so-called packed form before transmitting the information on the communication path, such as a radio channel. Such methods are based e.g. on the idea that elements recurring in the information to be transmitted are searched for and allotted a suitable index. Thus the recurrent element is replaced by the index having a shorter form, whereby the quantity of information to be transmitted is reduced. Such recurrent elements can be e.g. combinations of letters recurring in a text, such as abc. In the conventional presentation form of 8 bits, 24 (=3.times.8) bits are required for presentation of these letters. When this combination is allotted an index of e.g. 5 bits and all letter combinations abc recurring in the data to be transmitted are replaced by this index, 19 (=24-5) bits are saved for each letter combination to be replaced. Further, a method can be used where, if the same character recurs several times in succession, e.g. four subsequent numbers 0, these can be replaced by indicating the character to be repeated and the number of recurrences of this character. Thus the packing density is higher, as long as these are recurring.
The methods described above are effective in situations where a large quantity of data, such as different types of files, can be subjected to packing. If the quantity of information to be packed at a time is small, such as in the transmission of an access burst, using the above-described methods may result in increasing the quantity of information to be transmitted. For instance when only a few bits are to be packed, the methods described above are not usable.
In a European patent application EP-535 812 there is provided a method of receiving data encoded to permit forward error correction wherein selected elements of a set of original data elements relating to a first message are substituted by data elements relating to a second message. The first and second messages are two different messages like traffic channel message (TCH) and Fast Associated Control Channel message (FACCH). This method does not increase the length of information that can be sent in a burst.
Upon expanding present mobile communication network services, it will be necessary that the receiving equipment required for implementing transmission of a larger quantity of information is substantially not different from the equipment used in applying previous services. Consequently, e.g. the implementation of a base station could be substantially the present one, with the exception of reasonable software changes.